1. Field of the Invention
The present invention is directed to a method for operating of a magnetic resonance apparatus having a radio frequency system for emitting radio frequency excitation pulses and/or for receiving magnetic resonance signals, and having a gradient system for generating chronologically variable magnetic gradient fields, whereby both systems are activated dependent on time-variant signals that are made available in the form of discrete signal values.
2. Description of the Prior Art
A multitude of complex, time-variant signals are required for the operation of a magnetic resonance apparatus for imaging as well as for spectroscopy. First, time-variant and spatially dependent magnetic fields must be superimposed on a uniform field in three spatial directions. These so-called gradient fields are generated by time-variant currents, whereby the required currents have different function curves. Further, time-variant radio frequency pulses are required for the execution of the magnetic resonance. A sinusoidal carrier signal is thereby modulated with an envelope that is determined by one or more functions. Chronologically variable control signals and trigger pulse sequences are required in addition to the function signals for generating the gradient fields and the radio frequency excitation pulses. Included among these are, for example, digital values that determine the frequency and phase relation of the radio frequency excitation pulses, turn-on and turn-off signals, trigger time intervals for function generators that generate the aforementioned, complex function signals, and similar digital values at non-equidistant intervals.
Various methods are known that offer the discrete signal values, including the control and function signals, at the correct time in the magnetic resonance apparatus. A common factor of these known methods is that they all emit the control and function signals in digital form. Digital-to-analog converters with following amplifiers that belong to the systems then generate the physical (actual) signals that are supplied respectively to the gradient coils or radio frequency antenna.
In one such known method, the discrete signal values required for the operation of the magnetic resonance apparatus are sorted in a memory that is known by the designation "control word memory." The signal values are read out for the operation and are supplied to the system. This method requires a large amount of memory space. New sequences can only be implemented by modifying the memory content and the read-out procedure.
Memory space can be saved by using a method disclosed in German published application 38 02 082 and WO 86 04 702 wherein a time duration is attached to the control words or to the signal values. The relatively long times wherein a signal value remains unmodified are thus bridged. The time duration can be stored in the control word memory itself or in a separate time duration memory.
European published application 0 195 670 discloses a method wherein discrete signal values are deposited in a memory before a measurement or before the implementation of a magnetic resonance experiment and are read out by a read-out control for the measurement. The read-out control is operated by information from a control word memory.
In a further known method, the signal values are calculated on-line with a fixed program and variable data. The synchronization again ensues with control signals from the control word memory. Compared to the methods set forth above, this method saves memory space. Given modified or new sequences, the data structure and the program must be matched to the modifications. Mixed forms of these latter two methods are likewise possible.
The format and the structure of a control system with a control word memory is also described in "Bildgebende Systeme fur die medizinische Diagnostik", by Erich Krestel, Siemens AG, 2nd. Edition, 1988, Chapter 10.2.4 "Anlagensteuerung", pp. 528-534.
The article "Word Programmer for NMR" by G. Danese, D. Dotti, H. G. Jian, E. Braschi, P. Confrancesco, M. Villa, in Rev. Sci. Instrum., Vol. 57, No. 7, July 1986, pp. 1349-1353 discloses the generation of control signals as individual or discrete bit signals with the assistance of simple commands using a hard-wired logic.